1. Field of the Invention
The present invention pertains to the field of video communication systems. More particularly, this invention relates to a method for selecting between intra frame video data encoding and inter frame video data encoding in a video communication system.
2. Background
Prior video communication systems commonly employ data compression techniques to enable the real-time transfer of digitized video images over a relatively low bandwidth communication line. Typically, such prior video systems perform digital sampling on a video signal generated by a video camera. Such digital sampling provides an incoming digital video bit stream from the video camera that indicates the content of the corresponding image scene.
Such a prior video system usually encodes the incoming digital video bit stream according to a variety of available data compression techniques to generate compressed video data. The compressed video data is typically transmitted to a remote video system in a video conferencing system via a communication link. Typically, a remote video system in such a video conferencing system receives the compressed video data over the communication link and decodes the compressed video data to reconstruct a decompressed version of the original digital video bit stream.
Such video data compression techniques also enable compressed data storage of video images into a mass storage device such as a disk device or a CD ROM device. Such a video system retrieves the compressed video data from the mass storage device and then typically reconstructs a decompressed version of the original digital video bit stream to drive a display device. Such compressed video data storage increases the amount of video information stored in a given area of mass storage media, thereby increasing the effective data densities of mass storage devices such as disk devices and CD ROM devices.
The original digital video bit stream sampled by such a video system typically comprises a series of video frames. Each video frame usually comprises an array of pixel data that includes color or intensity values representing the original image scene sensed by the video camera. Typically, video data compression techniques are designed to minimize the size of the compressed video frames while maintaining the best possible reconstructed video image as the compressed video frames are decoded.
Prior video systems commonly encode each incoming video frame as either an inter frame or an intra frame. Typically, an inter frame is an incoming video frame encoded relative to a preceding video frame in the series of incoming video frames. An intra frame is typically an incoming video frame encoded without a relation to any other video frame in the series of incoming video frames.
Typically, inter frame encoding is more efficient than intra frame encoding for image scenes that do not change rapidly. An encoded inter frame for a current video frame usually contains difference values that indicate differences between the pixel values of the current video frame and the corresponding pixel values of the previous video frame. Such difference values are commonly zero for consecutive video frames of a relatively constant image scene. Such zero difference values typically increase the effectiveness of data compression techniques.
On the other hand, inter frame encoding is typically less efficient than intra frame encoding if the image scene exhibits large changes between consecutive video frames. Typically, an intra frame encoding of a current video frame contains mostly non zero pixel values for all pixels of the image scene. Such a large amount of non zero values usually decrease the effectiveness of data compression techniques.
Therefore, such a video system should advantageously select whether to encode each video frame as an inter frame or an intra frame on a frame by frame basis. Some prior video systems render such an intra frame versus inter frame decision by calculating a root mean squared error measure for consecutive incoming video frames. Such a root mean squared error measure typically indicates a relative difference between a current incoming video frame and a preceding video frame. Typically in such systems, intra frame encoding is selected if the root mean squared error measure is above a predetermined threshold value.
Such root mean squared error measures usually provide a sufficient indication of the differences between a current video frame and a preceding video frame and therefore a sufficient indication of the effectiveness of inter frame encoding. Unfortunately, such root mean squared error measures fail to provide an indication of the efficiency of the intra frame encoding. As a consequence, such a prior system may select an intra frame encoding for a target video frame based upon the root mean squared error measure even though an intra frame encoding would generate even more compressed video data than an inter frame encoding.
Unfortunately, such poor inter frame versus intra frame decisions in such systems usually cause inefficient data compression, thereby increasing the communication bandwidth required by a video conferencing system. Such high bandwidth communication increases the cost of video conferencing. In addition, poor inter frame versus intra frame decisions increase the mass storage area required to store the compressed video frames, thereby increasing the cost of video data storage.